[time-nuts] Advice on NTP server needed

Attila Kinali attila at kinali.ch
Mon Jun 13 22:59:52 UTC 2011

On Mon, 13 Jun 2011 15:37:13 -0700
Chris Albertson <albertson.chris at gmail.com> wrote:

> For NTP the issue is different.   The PPS signal coming from the GPS
> needs to go into a hardware serial port so the PPS handler sees a very
> low but more importantly a predictable latency.    The PPS depends on
> a very specif interface to the hardware interrupt controller.

The interrupt latency for USB is predictable. You can read out the
bInterval setting of the interrupt endpoint of your serial converter
(use something like lsusb or similar), this is the maximum poll interval
in ms (for USB FS devices). Most devices i've seen, set this to 1

Given that USB port is fully loaded, then you will get an average
delay of 0.5ms (uniform distribution). With an unloaded USB port,
this will be much better. (for exact values, check the source of
your OS/driver or use an USB debugger like the beagle to measure it)

NTP will filter out the jitters of the PPS input using its control
loop, so in the end, all that's left is the average delay as offset.

Considering that anything below 1ms is negligible when comming to NTP,
then i'd say that a USB serial port is good enough. Of course, 1ms timing
uncertainty is not time-nuts class, but then again, ntp isnt
time-nuts class ;-)

			Attila Kinali
Why does it take years to find the answers to
the questions one should have asked long ago?

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